The Wreck of the old '485..

My Tek 485 had been sleeping on the shelf and I needed to take it to a friend's house to help him debug a problem. Checking it out before I left, I found it did not light up -- no power, to pilot light, no trace, nothing.

So, with the help of my friend, Dick, WB6JDH, I took it apart and did a preliminary visual on the back plane, the line board, the switcher board and the PS board. The two 430-ohm resistors (R1812 and R8113) were burnt out so I replaced them with two 2-watt 470-ohm resistors on the line board.

All else looked fine. After cleaning out some carbon and dust, pulling the "start" transistor and then the "stop" transistor on the PS board and bringing the unit up on a variac with each, we got the power supply to work -- but experienced some smoking and burning during the process.

Cleaning out further carbon and burned areas revealed no seemingly bad components so we brought the unit fully up under power and it seemed stable. However, attempting to view a trace on the scope revealed that the vertical position controls did not work and I was unable to get a dual trace. I was pretty careful with the setup and triggering so I do not thin thee was a cockpit error. Another anomaly was a distinct PS snapping when the "ADD" button was pressed.

Here's the video:

Resolved!
A tantalum cap on the A7 board (near U650 and the 15V test point) shorted and took out the associated RF choke. We swapped the A7 board with one from a donor 485. Turns out that one was bad as well. Q374 and Q384 were both bad and were replaced with matched 2N3904s. Was easy to do because most semiconductors are socketed on that board. But that was karma because removing and replacing the A7 board WAS a bitch! The delay line was soldered on and a semiconductor on the board was bolted on a heat sink to the frame below. Getting its leads through the two holes in the sub-chassis was a delicate operation -- especially on a 100-degree day with 90% humidity.

I deserve NONE of the credit for solving this problem! Dick, WB6JDH, [literally] sweated through the debugging process and suggested we swap boards from the old donor unit. I just dd the donkey work and took copious notes.

I also treated us to burgers and dogs afterwards.

Resolution Resolved
Actually, the resolution was a false positive and the malady returned. (And, truth be told, who's to say it won't return in the future?) But, in tracing the origin of the horizontal position controls back to their source on the power supply board, I found all of the components in each of the position controls to be within spec and performing correctly. However, it was noted that the cable plugs along the top of the PS board were unseated and, after re-seating them, the scope seemed to function correctly. So, as I said, the problem might re-occur but at least I'll know how to troubleshoot it.

Stay tuned..or don't!

-73-